1. Field of the Invention
The present invention relates to an illumination optical system suitable for manufacturing a semiconductor and an exposure apparatus including the same system.
2. Related Background Art
A reduction projection type exposure apparatus has hitherto been employed for manufacturing a semiconductor element such as LSI (large scale integrated circuits) and VLSI (very large scale integrated circuits) consisting of hyperfine patterns. A good deal of endeavors have continued for transferring much hyperfiner patterns. An attempt to increment a numerical aperture (hereinafter abbreviated to NA) of a projection optical system has been made with a reduction in wavelength of exposure light in order to correspond to the pattern micronization described above. There has been actualized a projection optical system exhibiting NA=0.5 or above.
Then, optimization of an illuminating condition is of much importance for the actual projection exposure wherein the projection optical system having such a large numerical aperture NA is used. Under such circumstances, for instance, Japanese Patent Laid-Open Application No. 59-155843 proposes such an arrangement that an NA ratio between two optical systems is controlled to obtain a proper balance between resolution and contrast With respect to a predetermined pattern, and this involves controlling a so-called .sigma.-value corresponding to the ratio of NA of an illumination optical system to NA of the projection optical system.
According to the apparatus proposed therein, a variable aperture stop having a variable opening is disposed in an exit position of a fly eye lens in which a secondary illuminant (light source) image is formed in the illumination optical system. A size of the illuminant image is controlled by changing a size of the opening of this variable aperture stop. Namely, the .sigma.-value corresponds to a ratio of size of the illuminant image, formed on the pupil of the projection optical system, of the illumination optical system to a size of the pupil of the projection optical system. Therefore, NA of the illumination optical system is substantially changed by making the illuminant image size variable with a variation in terms of light shielding degree of the actually formed illuminant image. The illuminating condition, i.e., the .sigma.-value is thereby optimized to set the resolution and the contrast with respect to the predetermined pattern in an optimum state in accordance with a degree of micronization of the pattern undergoing the projection exposure.
Generally, a higher illuminance on an irradiated surface (mask or reticle) is demanded to improve the throughput when effecting exposure-printing of the hyperfine pattern in this type of illumination optical apparatus.
In the conventional apparatus mentioned above, however, a size of the opening of the variable aperture stop is reduced to optimize the .sigma.-value as the illuminating condition under which the resolution and the contrast with respect to the predetermined pattern are in the optimum state in accordance with the degree of the micronization of the pattern subjected to the projection exposure. In this case, a loss in light quantity augments because of a peripheral portion of the secondary illuminant image being shielded from the light by the variable aperture stop. More specifically, when the .sigma.-value is maximized (the aperture of the variable aperture stop is maximized), the maximum illuminance on the irradiated surface (mask or reticle) is obtained. Nevertheless, there exists a critical defect in which the throughput inevitably decreases due to a decline of the illuminance on the irradiated surface when smaller than that value (the aperture of the variable aperture stop is reduced).